Compound semiconductor epitaxial substrate and manufacturing method thereof

ABSTRACT

A compound semiconductor epitaxial substrate having a pseudomorphic high electron mobility field effect transistor structure including an InGaAs layer as a strained channel layer and an AlGaAs layer containing n type impurities as a front side electron-donating layer, wherein said substrate contains an InGaP layer in an orderly state on the front side of the above described InGaAs layer as the strained channel layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Divisional Application of U.S. application Ser. No. 10/545,295filed Aug. 11, 2005, which is a national stage of PCT/JP2004/001117filed Feb. 4, 2004, which claims priority to Japanese Application No.2003-033466 filed Feb. 12, 2003. The entire disclosures of the priorApplications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a compound semiconductor epitaxialsubstrate for pseudomorphic high electron mobility transistorscomprising III-V compound semiconductors and a manufacturing methodthereof.

BACKGROUND ART

High electron mobility field effect transistors (hereafter HEMT) havebeen used as an important component of radiofrequency communicationequipment. The HEMT is characterized by the selectively dopedheterostructure consisting of different materials for theelectron-donating layer (dope layer) and the channel layer in whichelectrons travel. In this heterostructure, electrons supplied from ntype impurities in the electron-donating layer collects in the potentialwells formed on the channel side of the heterojunction interface due tothe difference of electron affinity of materials constituting theheterojunction, resulting in the formation of two dimensional electrongas. Thus n type impurities supplying electrons are in theelectron-donating layer, and because the electrons supplied from hereseparate ionization impurities spatially from electrons as they travelthrough the high-purity channel, the two dimensional electron gas in thechannel is not scattered by ionization impurities and has high electronmobility.

While the HEMT is normally manufactured using an epitaxial substratelaminating thin film crystal layers having given electroniccharacteristics on a GaAs single crystal substrate so as to possessgiven structure, it is required to control the thin film crystal layerwhich forms the HEMT structure on the order of monoatomic layers so thatthe channel can have high electron mobility. Thus, to manufacture anepitaxial substrate having the HEMT structure, the molecular beamepitaxy (hereafter referred to as MBE) method or the metal-organicchemical vapor deposition (hereafter referred to as MOCVD) method hasbeen used conventionally.

Of these methods, the MOCVD method, in particular, which involves usingorganometallic compounds or hydrides of atomic species constituting theepitaxial layer as raw materials and growing crystals on the substratethrough thermal decomposition, has been widely used in recent yearsbecause of its wide applicability range and fitness for precise controlof crystal composition and the film thickness.

III-V compound semiconductor materials widely used for these epitaxialsubstrates include GaAs and AlGaAs because they allow matching thelattice constant with given composition and various types ofheterojunction are possible while keeping good crystallinity. However,because it is necessary to increase the electron mobility of the channellayer in order to improve the performance of HEMT, InGaAs has been usedin recent years as a material for the channel layer instead of GaAsbecause it is not only superior in electron transport property but itcan also change energy gaps dramatically according to the In compositionand contain two dimensional electrons effectively. In addition, AlGaAsor GaAs can be used as a material to be combined with InGaAs.

Because lattice matching for GaAs was impossible, InGaAs could not beused formerly to obtain an epitaxial substrate having sufficientphysical property. However, since reliable heterojunction was foundpossible even in the lattice mismatch system without causing a reductionin crystallinity such as dislocation if the mismatching is within thethreshold of elastic deformation, efforts have been made towardspractical use.

The threshold of film thickness of strained crystalline layer in suchlattice mismatch system is given as a function of crystal layercomposition, and in the case of the InGaAs layer for the GaAs layer, forexample, the theoretical formula of Mathews is disclosed in J. CrystalGrowth, 27 (1974), p. 118 and J. Crystal Growth, 32 (1976), p. 265.These theoretical formulas have been found almost correct inexperiments.

In addition, JP-A-6-21106 discloses a technique to improve electronmobility by optimizing the In composition of the InGaAs strain layer andthe film thickness of the InGaAs layer used for the channel layer of thep-HEMT structure using a given relational expression. Actually, anInGaAs layer with In composition of 0.20 and film thickness of about 13nm has been put to practical use as an InGaAs strained channel layerthat allows epitaxial growth without reducing crystallinity.

By using an epitaxial growth substrate configured to use such InGaAslayer for the channel layer part of conventional HEMT in which twodimensional electrons flow, electron devices have been fabricated thathave higher mobility and superior noise characteristics compared toconventional ones. The HEMT using the InGaAs layer for the channel layerin which two dimensional electrons flow is referred to as apseudomorphic high electron mobility transistor (hereafterpseudomorphic-HEMT or p-HEMT).

In p-HEMT, a layer called a space layer is usually formed between thestrained channel layer, InGaAs layer, and the front sideelectron-donating layer as the layer to reduce the effect of impurityscattering due to the front side electron-donating layer on theelectrons flowing in the channel layer. Furthermore, a layer to installa gate electrode of transistor generally referred to as a gate barrierlayer or Schottky layer is formed on the surface side of the front sideelectron-donating layer. For these space layers and gate barrier layers,GaAs layers or AlGaAs layers have been used conventionally.

In addition, in p-HEMT, a GaAs or AlGaAs layer is usually formed as theelectron-donating layer. However, an InGaP layer joined to a GaAs orAlGaAs layer in a lattice matching manner has been also used.

However, using a GaAs or AlGaAs layer for the space or gate barrierlayer is problematic; GaAs has too small a band gap to allow gatewithstand voltage for transistor gates, and AlGaAs has a problem in thatincorporation of impurities results in the loss of crystallinity andsurface state stability.

In addition, conventional p-HEMT structure required a layer rich indopant as an electron-donating layer in order to achieve an amount oftwo dimensional electron gas required in the channel layer to improvethe current value of transistors. However, for the reasons describedabove, it was difficult to further improve transistor performancebecause the crystallinity of the electron-donating layer decreased dueto excess dopant and the withstand voltage of the gate decreased.

As means of solving these problems, a configuration designed to lowerthe dopant concentration of the front side electron-donating layer andthicken its film thickness, or in the case of a double hetero structure,a configuration designed to lower the dopant concentration of the frontside electron-donating layer and increase the dopant concentration ofthe back side has been proposed.

However, even if the configuration proposed above is employed in anepitaxial substrate of p-HEMT structure, it is difficult to employ anelectron-donating layer with a low dopant concentration to achieve ahigh two dimensional electron gas concentration and obtain an epitaxialsubstrate of p-HEMT structure having good transistor characteristicssuch as pinch off characteristics if GaAs or AlGaAs is used for the gatebarrier layer.

In this view, for the p-HEMT used for various mobile equipment such ascell phones, improving gate withstand voltage and pinch offcharacteristic is required, and it is necessary to use anelectron-donating layer with a low dopant concentration to increase twodimensional electron gas concentration to improve the characteristics ofelectronic devices. However, the above described conventionaltechnologies are not sufficient to meet these needs.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a compoundsemiconductor epitaxial substrate that can solve the above describedproblems with conventional technologies, and a manufacturing methodthereof.

It is another object of the present invention to provide a compoundsemiconductor epitaxial substrate having a p-HEMT structure that isdesigned to improve gate withstand voltage and pinch offcharacteristics, and a manufacturing method thereof.

It is another object of the present invention to provide a compoundsemiconductor epitaxial substrate having a p-HEMT structure that employsan electron-donating layer with a low dopant concentration to achieve ahigh two dimensional electron gas concentration and has high electronmobility, and a manufacturing method thereof.

To solve the above described problems, in one aspect of the presentinvention, an orderly InGaP layer was established on the front side(opposite side of the substrate) of the InGaAs layer, which is thestrained channel layer of p-HEMT, and, because the interface state ofthe InGaP layer was low and interface charge was generated, anelectron-donating layer with a low dopant concentration was used tofabricate an epitaxial substrate having the HEMT structure with bothhigh two dimensional electron gas concentration and high electronmobility. Gate withstand voltage can be improved if the front sideelectron-donating layer of p-HEMT can be grown at low dopantconcentration, because electric field intensity between the gateelectrode and channel decreases.

Here, the InGaP layer in orderly state can be defined by band gap valuesfor InGaP. The band gap of InGaP varies depending on the growthtemperature when the InGaP layer grows, and the InGaP layer will be inorderly state when the band gap is minimal. Band gap values for theInGaP in orderly state were 1.84 eV to 1.85 eV. In semiorderly state inwhich the band gap value is larger than this, the effect of interfacecharge on the increase in dopant efficiency is observed, though it issmall, because the generation of interface charge continues.

In a first aspect of the present invention, a compound semiconductorepitaxial substrate is proposed that has a pseudomorphic high electronmobility transistor structure including an InGaAs layer as the strainedchannel layer and an AlGaAs layer containing n type impurities as thefront side electron-donating layer, the substrate comprising an InGaPlayer in an orderly state on the front side of the above describedInGaAs layer which is the strained channel layer.

In a second aspect of the present invention, a compound semiconductorepitaxial substrate is proposed that has a pseudomorphic high electronmobility transistor structure including an InGaAs layer as the strainedchannel layer and an InGaP layer containing n type impurities as thefront side electron-donating layer, the substrate comprising an InGaPlayer in an orderly state on the front side of the above describedInGaAs layer which is the strained channel layer.

In a third aspect of the present invention, a method of manufacturing acompound semiconductor epitaxial substrate of the above described firstor second aspect is proposed, the method comprising the epitaxial growthof each compound semiconductor layer using the metal-organic chemicalvapor deposition (MOCVD) method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a layer structure view showing an embodiment of the epitaxialsubstrate according to the present invention;

FIG. 2 is a layer structure view showing the first example of theepitaxial substrate according to the present invention;

FIG. 3 is a layer structure view showing the second example of theepitaxial substrate according to the present invention; and

FIG. 4 is a layer structure view showing the third example of theepitaxial substrate according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention is described in detail below byreference to the drawings.

FIG. 1 is a layer structure view showing an embodiment of an epitaxialsubstrate of the p-HEMT structure according to the present invention. InFIG. 1, reference numeral 1 denotes a GaAs monocrystal substrate, andreference numeral 2 denotes a buffer layer formed on the GaAsmonocrystal substrate 1. Reference numeral 3 denotes the back sideelectron-donating layer doped with n type impurities that is formed as an-AlGaAs layer, and a back side space layer (i-AlGaAs layer) 4 is formedon the back side electron-donating layer 3. Reference numeral 5 denotesa channel layer in which two dimensional electrons flow to form twodimensional electron gas, and represents a strained channel layer formedas an i-InGaAs layer.

On the channel layer 5 are formed a front side space layer 6 consistingof an i-AlGaAs layer and a front side electron-donating layer 7 formedas a n-AlGaAs layer, and a gate barrier layer 8 consisting of an i-InGaPlayer in orderly state is formed in the upper part (on the opposite sideof the GaAs monocrystal substrate 1) of the front side electron-donatinglayer 7.

Although an i-InGaP layer in orderly state is formed as the gate barrierlayer in FIG. 1, the i-InGaP layer may be installed as a space layer ora layer further on the front side of the gate barrier layer if it is onthe front side of the i-InGaAs layer which is a strained channel layer.In addition, the i-InGaP layer 8, i-AlGaAs layer 6, i-InGaAs layer 5 andi-AlGaAs layer 4, all of which are shown to be of the i type in FIG. 1,may also be of the n or p type.

Because the epitaxial substrate in FIG. 1 is formed as described above,electrons are supplied to the channel layer 5 from the back sideelectron-donating layer 3 through the back side space layer 4 as well asfrom the front side electron-donating layer 7 through the front sidespace layer 6. As a result, a high-density two dimensional electron gasis formed in the channel layer 5. Because the interface state is low asan i-InGaP layer in orderly state is formed on the front sideelectron-donating layer 7 as the gate barrier layer 8, and due to theeffect of interface charge, the two dimensional electron gasconcentration in the channel layer 5 can be maintained at high levelseven if the dopant concentration in the electron-donating layer is low.Thus, gate withstand voltage can be improved because a low dopantconcentration can be used and the electric field intensity between thegate electrode and channel can be lowered. In addition, because ani-InGaP layer with a large band gap is formed as the gate barrier layer8, an improvement in gate withstand voltage can be expected compared tothe gate barrier layer using GaAs or AlGaAs. Because two dimensionalelectron gas with an increased concentration is also achieved with thesame amount of dopant as before, the present invention is advantageousfor use in p-HEMT used with a high driving current value.

The embodiment in FIG. 1 shows a case in which an AlGaAs layercontaining n type impurities is used as the front side electron-donatinglayer 7. However, the front side electron-donating layer 7 is notlimited to this configuration, and the above described AlGaAs layer maybe replaced by an InGaP layer containing n type impurities, which isexpected to have similar effects as the configuration shown in FIG. 1.If an InGaP layer is used for the electron-donating layer, no DX centeris formed that may affect the temperature characteristics of transistorscompared to the AlGaAs layer, and doping with a high concentration ofdopant will be possible.

Furthermore, when a n-InGaP layer is used as the front sideelectron-donating layer 7, the AlGaAs layer may be replaced by ani-InGaP layer for the front side space layer 6. Thus, if an i-InGaPlayer is used for the space layer, a space layer with good crystallinitycan be formed that incorporates fewer impurities, such as C and O, thanwhen an i-AlGaAs layer is used.

Obviously, the same effect as the configuration shown in FIG. 1 isprovided with p-HEMT of the single heterostructure lacking the back sideelectron-donating layer 3 and the back side space layer 4.

While the structure in which the channel layer is an InGaAs layer hasbeen explained, it is clear that similar effects are provided even whenIII-V compound semiconductor layers, such as a GaAs layer, AlGaAs layerand InGaP layer, are used as the channel layer.

An example of a method of fabricating an epitaxial substrate of thelayer structure shown in FIG. 1 is described below. First a GaAsmonocrystal substrate 1 is prepared. The GaAs monocrystal substrate 1 isa high-resistance semi-insulating GaAs monocrystal substrate andpreferable fabrication methods include the LEC (Liquid EncapsulatedCzochralski) method, VB (Vertical Bridgeman) method and VGF (VerticalGradient Freezing) method. Whichever method is used, a substrate with agradient of about 0.05 to 10° from one crystallographic plane directionis prepared.

The surface of the GaAs monocrystal substrate 1 prepared as describedabove is subjected to degreasing/cleaning, etching, washing and drying,then the substrate is placed on the heating table of a crystal growthfurnace. Application of heat is started after having substituted highpurity hydrogen for the inside of the furnace sufficiently. Arsenic rawmaterial is introduced inside the furnace after reaching stable moderatetemperature. Gallium raw material is then introduced when producing aGaAs layer. Gallium and aluminum raw materials are introduced inaddition to the arsenic raw material when producing an AlGaAs layer.Gallium and indium raw materials are introduced in addition to thearsenic raw material when producing an InGaAs layer. Desired layeredstructure is developed by controlling the feeding rate and time of eachraw material. Lastly, the feeding of each raw material is stopped tostop crystal growth, and after cooling the layered epitaxial substrate,as shown in FIG. 1, is taken out of the furnace to complete crystalgrowth. The substrate temperature during crystal growth is usually about500 to 800° C.

The epitaxial substrate of the layer structure shown in FIG. 1 can befabricated by the MOCVD method. An advantage of using the MOCVD methodis that organometallic compounds or hydrides of atomic speciesconstituting the epitaxial layer can be used as raw material.

Actually, arsenic trihydride (arsine) is usually used as an arsenic rawmaterial for epitaxial growth; however, alkyl arsine in which a hydrogenatom of arsine is substituted by an alkyl group having one to fourcarbon atoms can be used. As raw material for gallium, aluminum andindium, trialkyl or trihydride compounds of respective metal atoms aregenerally used to which an alkyl group having one to three carbon atomsor hydrogen atoms is attached.

For n-type dopant, a hydride or an alkyl compound having an alkyl groupwith one to three carbon atoms of silicone, germanium, tin, sulfur,selenium, etc. can be used.

The following examples and comparative examples illustrate the presentinvention in detail, but are not intended to limit the scope of theinvention. In addition, the layer structure of the epitaxial substrateshown in examples is for measuring the characteristics of epitaxialsubstrate, and the actual epitaxial substrate for p-HEMT is configuredto have additional layers such as n-GaAs and n-AlGaAs layers. It isobvious, however, that such actual epitaxial substrate for p-HEMT willhave similar characteristics to those of the examples described below.

Example 1

The layer structure shown in FIG. 2 was created on a semi-insulatingGaAs monocrystal substrate, which was prepared by the VGF method,through epitaxial growth by the MOCVD method using a vacuum barrel typeMOCVD furnace.

In FIG. 2, reference numeral 21 denotes a GaAs monocrystal substrate,and reference numerals 22 to 25 denote buffer layers formed on the GaAsmonocrystal substrate 21. Here, the buffer layers 22 to 25 are formed as200 nm-thick i-GaAs layer, 250 nm-thick i-Al_(0.25)Ga_(0.75)As layer,250 nm-thick i-GaAs layer and 200 nm-thick Al_(0.24)Ga_(0.76)As layer,respectively.

Reference numeral 26 denotes the back side electron-donating layer dopedwith n type impurities at 4×10¹⁸/cm³ that is formed as a 4 nm-thickn-Al_(0.24)Ga_(0.76)As layer, and back side space layers 27 and 28 areformed in this order on the back side electron-donating layer 26. Here,the back side space layer 27 is a 3 nm-thick i-Al_(0.24)Ga_(0.76)Aslayer and the back side space layer 28 is a 5 nm-thick i-GaAs layer.Reference numeral 29 is a channel layer in which two dimensionalelectrons flow to form two dimensional electron gas, and represents astrained channel layer consisting of a 7.5 nm-thicki-In_(0.30)Ga_(0.70)As layer.

Reference numerals 30 and 31 denote front side space layers. Here, thefront side space layer 30 is a 5 nm-thick i-GaAs layer and the frontside space layer 31 is a 3 nm-thick i-Al_(0.24)Ga_(0.76)As layer.

Reference numeral 32 denotes a front side electron-donating layerconsisting of a 10 nm-thick n-Al_(0.24)Ga_(0.76)As layer doped with ntype impurities at 4×10¹⁸/cm³. A 28 nm-thick i-In_(0.483)Ga_(0.517)Player in orderly state is formed on and in close contact with the frontside electron-donating layer 32 as a gate barrier layer 33.

Raw materials of the Group III elements included trimethylgallium,trimethylaluminum and trimethylindium, and as raw materials of the GroupV elements, arsine and phosphine were used. For n-type dopant, silanediluted to 0.005% in hydrogen was used. High purity hydrogen was used ascarrier gas for the raw materials, and epitaxial growth was promotedunder the conditions of reactor pressure of 0.1 atm, growth temperatureof 650° C. and growth rate of 3 to 1 μm/hr. The In composition of 0.483was used for the gate barrier layer 33, because this condition favoredlattice matching to the GaAs and AlGaAs layers. The InGaP layer inorderly state was thus produced.

The compound semiconductor epitaxial substrate prepared as describedabove so as to have the layer structure shown in FIG. 2 was subjected tohole measurement by the Van der Pauw method. Results show that the twodimensional electron gas concentration in the channel layer 29 at roomtemperature, (300 K) was 2.85×10¹²/cm², electron mobility at roomtemperature (300 K) was 7830 cm²/V·s, the two dimensional electron gasconcentration at 77 k was 2.85×10¹²/cm², and electron mobility at 77 kwas 27400 cm²/V·s. The amount of dopant supplied to the front sideelectron-donating layer 32 during manufacture was as small as 51.9 cc.In addition, CV measurement using an Al Schottky electrode was performedon this structure; the pinch off voltage at the residual carrier densityof 1×10¹⁵/cm³ was −2.91 V.

Comparative Example 1

An epitaxial substrate was prepared by the MOCVD method in the samemanner as Example 1, in which the i-In_(0.483)Ga_(0.517)P layer inorderly state constituting the gate barrier layer 33 of the layerstructure of Example 1 shown in FIG. 2 was replaced by a 28 nm-thicki-Al_(0.24)Ga_(0.76)As layer, the n type impurity concentration in theelectron-donating layers 26 and 32 was specified to be 4.5×10¹⁸/cm³, andother layers were as in Example 1.

Hole measurement by the Van der Pauw method was performed on the layerstructure of Comparative Example 1, which is of a conventional p-HEMTstructure. Results show that the two dimensional electron gasconcentration in the channel layer 29 at room temperature (300 K) was2.84×10¹²/cm², electron mobility at room temperature (300 K) was 7940cm²/V·s, the two dimensional electron gas concentration at 77 k was2.89×10¹²/cm², and electron mobility at 77 k was 27800 cm²/V·s, and theamount of dopant supplied to the front side electron-donating layer 32was 58.9 cc. In addition, CV measurement using an Al Schottky electrodewas performed on this structure; the pinch off voltage at the residualcarrier density of 1×10¹⁵/cm³ was −2.52 V.

When the amount of dopant supplied to the front side electron-donatinglayer 32 was reduced to the same amount of 51.9 cc as Example 1, the twodimensional electron gas concentration in the channel layer 29 at roomtemperature (300 K) was 2.28×10¹²/cm², resulting in a decrease of0.57×10¹²/cm² in the two dimensional electron gas concentration at roomtemperature (300 K) compared to Example 1.

In Example 1 according to the present invention, the values for the twodimensional electron gas concentration and electron mobility were almostequal to those for Comparative Example 1, which is a conventionalexample, while the amount of dopant supplied to the front sideelectron-donating layer 32 was smaller by 10% or more than that inComparative Example 1. Thus by using the configuration of Example 1, thevalues for the two dimensional electron gas concentration and electronmobility that were equivalent to those for conventional p-HEMT could beobtained even when a smaller amount of dopant was supplied. Becauseequivalent two dimensional electron gas concentration and electronmobility could be thus achieved with a smaller supply of dopant, gatewithstand voltage can be improved without decreasing the driving currentvalue of transistors.

Example 2

The epitaxial substrate of the layer structure shown in FIG. 3 wasprepared by the MOCVD method in the same way as Example 1. The layerstructure of Example 2 shown in FIG. 3 differs from that of Example 1only in that the front side electron-donating layer 32B consisted of a10 nm-thick n-In_(0.483)Ga_(0.517)P layer with a carrier density of4×10¹⁸/cm³ containing n type impurities. Therefore, codes are assignedto the other layers in FIG. 3 that are identical with the codes of thecorresponding layers in FIG. 2, and are not described here.

Hole measurement by the Van der Pauw method was performed on the layerstructure of Example 2; results show that the two dimensional electrongas concentration in the channel layer 29 at room temperature (300 K)was 2.87×10¹²/cm², electron mobility at room temperature (300 K) was7840 cm²/V·s, the two dimensional electron gas concentration at 77 k was2.85×10¹²/cm², and electron mobility at 77 k was 29100 cm²/V·s, and theamount of dopant supplied during manufacture to the front sideelectron-donating layer was 78.5 cc. In addition, CV measurement usingan Al Schottky electrode was performed on this structure; the pinch offvoltage at the residual carrier density of 1×10¹⁵/cm³ was −3.07 V.

Comparative Example 2

Comparative Example 2 was conducted by the MOCVD method in accordancewith Example 2, which had the same layer structure as Example 2 shown inFIG. 3 except that a 28 nm-thick i-Al_(0.24)Ga_(0.76)As layer wassubstituted for the gate barrier layer 33 of Example 2 shown in FIG. 3and the n type impurity concentration in the electron-donating layers 26and 32B was specified to be 4.5×10¹⁸/cm³. The layer structure of thiscomparative example 2 is of a conventional p-HEMT structure.

Hole measurement by the Van der Pauw method was performed on the layerstructure of Comparative Example 2; results show that the twodimensional electron gas concentration in the channel layer 29 at roomtemperature (300 K) was 2.88×10¹²/cm², electron mobility at roomtemperature (300 K) was 7860 cm²/V·s, the two dimensional electron gasconcentration at 77 k was 2.86×10¹²/cm², and electron mobility at 77 kwas 30100 cm²/V·s, and the amount of dopant supplied during manufactureto the front side electron-donating layer was 87.3 cc. In addition, CVmeasurement using an Al Schottky electrode was performed on thisstructure; the pinch off voltage at the residual carrier density of1×10¹⁵/cm³ was −2.63 V.

When the amount of dopant supplied to the front side electron-donatinglayer 32B was reduced to the same amount of 78.5 cc as Example 2, thetwo dimensional electron gas concentration in the channel layer 29 atroom temperature (300 K) was 2.30×10¹²/cm², resulting in a decrease of0.55×10¹²/cm² in the two dimensional electron gas concentration at roomtemperature (300 K) compared to Example 2.

In Example 2 according to the present invention, the values for the twodimensional electron gas concentration and electron mobility were almostequal to those for Comparative Example 2, which is a conventionalexample, while the amount of dopant supplied to the front sideelectron-donating layer 32 was smaller by 10% or more than that inComparative Example 2. Thus, by using the configuration of Example 2,values for the two dimensional electron gas concentration and electronmobility that were equivalent to those for conventional p-HEMT could beobtained even when a smaller amount of dopant was supplied. Becauseequivalent two dimensional electron gas concentration and electronmobility could be thus achieved with a smaller supply of dopant, gatewithstand voltage can be improved without decreasing the driving currentvalue of transistors.

Example 3

The epitaxial substrate of the layer structure shown in FIG. 4 wasprepared by the MOCVD method in the same way as Example 1. The layerstructure of Example 3 shown in FIG. 4 differed from that of Example 2only in that a 3 nm-thick i-In_(0.483)Ga_(0.517)P layer in orderly statewas replaced for the i-AlGaAs layer of the front side space layer 31B,the gate barrier layer 33A consisted of a 28 nm-thicki-Al_(0.24)Ga_(0.76)As layer, and the n type impurity concentration inthe electron-donating layers 26 and 32B was specified to be4.3×10¹⁸/cm³. Therefore, codes are assigned to the other layers in FIG.4 that are identical with the codes of the corresponding layers in FIG.3, and are not described here.

Hole measurement by the Van der Pauw method was performed on the layerstructure of Example 3; results show that the two dimensional electrongas concentration in the channel layer 29 at room temperature (300 K)was 2.89×10¹²/cm², electron mobility at room temperature (300 K) was7890 cm²/V·s, the two dimensional electron gas concentration at 77 k was2.83×10¹²/cm², and electron mobility at 77 k was 31200 cm²/V·s, and theamount of dopant supplied to the front side electron-donating layer was84.7 cc. In addition, CV measurement using an Al Schottky electrode wasperformed on this structure; the pinch off voltage at the residualcarrier density of 1×10¹⁵/cm³ was −2.80 V.

In Example 3 according to the present invention, the values for the twodimensional electron gas concentration and electron mobility were almostequal to those for Comparative Example 2, which is a conventionalexample, while the amount of dopant supplied to the front sideelectron-donating layer 32B was smaller by 3% more than that inComparative Example 2. Thus by using the configuration of Example 3, thevalues for the two dimensional electron gas concentration and electronmobility that were equivalent to those for conventional p-HEMT could beobtained even when a smaller amount of dopant was supplied. Becauseequivalent two dimensional electron gas concentration and electronmobility could be thus achieved with a smaller supply of dopant, gatewithstand voltage can be improved without decreasing the driving currentvalue of transistors.

INDUSTRIAL APPLICABILITY

The present invention provides an epitaxial substrate of the p-HEMTstructure having the best characteristic ever possible using InGaPmaterial that is superior in both crystalline purity and temperaturecharacteristics at the time of manufacture of electronic devices.

1. A compound semiconductor epitaxial substrate having a pseudomorphichigh electron mobility field effect transistor structure comprising anInGaAs layer as a strained channel layer, an AlGaAs layer containing ntype impurities as a front side electron-donating layer, and an InGaPlayer in an orderly state as a gate barrier layer, in this order,wherein the orderly state is a state where the band gap value for theInGaP layer is 1.85 eV or less.
 2. A method of manufacturing a compoundsemiconductor epitaxial substrate according to claim 1, comprisingepitaxial growth of each layer of the compound semiconductor epitaxialsubstrate using a metal-organic chemical vapor deposition (MOCVD)method.